Without conclusive evidence, what the intelligence agencies would call "confirmation" rather than "high confidence", we are left to speculate over alternative explanations. The main scenario that troubles me is that one of the more extreme Islamist rebel factions did it in order to trick Western powers into attacking the Assad regime. This rests mainly upon analysis of motives. The Assad regime lacks a clear motive. For it to have perpetrated the 21 August gas attack would seem to be taking much too great a strategic risk for much too little tactical advantage -- especially as the war has already been going its way. However, some extreme Islamist rebel factions, especially the Jabhat al-Nusra, are foreign led Al Qaeda operations that might not be at all upset over a chemical attack on positions held by rival, primarily Syrian nationalist, rebel factions. Indeed, for them that might be seen as all to the good.
With respect to means, rather than motive, there are
reports that seem credible, although largely ignored by most mainstream
Western media, that Al Qaeda cells busted in Iraq and in Turkey were producing or in possession of sarin. The "lack of flight activity or
missile launches" and rockets being used instead could be interpreted as
actually pointing more towards the rebels, who have such rockets, than
the Assad regime which might have preferred to use the more accurate
means of delivery so close to their own positions.
consider what the effect would be if there is a US attack upon the Assad
forces as a result of the 21 August gas attack if it is actually a
successful Al Qaeda engineered hoax. Not only does it not deter the
Assad regime from chemical attacks it isn't responsible for but it would
positively encourage additional chemical attacks by the actual
culprits. Shouldn't we have more than "high confidence" that we would
not be making such a terrible blunder?